In my experience building Qt apps for a major automotive manufacturer (and more recently, building a web app), the latter is much easier, even if you know C++ and Qt and have mediocre knowledge of web tech. If you don't know C++, then add 6-24 months to the Qt estimate, and expect it to have more bugs than an Appalachian motel.

How much QML in total application size did you use?

Our application largely predated QML, so none.